[Sync] Implement MemoryDumpProvider.

components/sync/syncable/parent_child_index.cc

Index: components/sync/syncable/parent_child_index.cc
diff --git a/components/sync/syncable/parent_child_index.cc b/components/sync/syncable/parent_child_index.cc
index c0db190fe76f8ea4b012ad28232ac01ea6f8035a..386a92d92140f4612df5cd6d9abcf5e9c249302f 100644
--- a/components/sync/syncable/parent_child_index.cc
+++ b/components/sync/syncable/parent_child_index.cc
@@ -167,6 +167,13 @@ const OrderedChildSet* ParentChildIndex::GetSiblings(EntryKernel* e) const {
   return siblings.get();
 }
 
+size_t ParentChildIndex::EstimateMemoryUsage() const {
+  using base::trace_event::EstimateMemoryUsage;
+  return EstimateMemoryUsage(parent_children_map_) +
+         EstimateMemoryUsage(model_type_root_ids_) +
+         EstimateMemoryUsage(type_root_child_sets_);
+}
+
 /* static */
 bool ParentChildIndex::ShouldUseParentId(const Id& parent_id,
                                          ModelType model_type) {